The teachings of the present invention can be practiced on any magnetic material having domains of such size that refinement thereof would produce significant core loss improvement, such as cube-on-face oriented electrical steel (designated (100) [001] by Miller's Indices) and cube-on-edge oriented silicon steels. For purposes of an exemplary showing the invention will be described in its application to improvements in the core loss of cube-on-edge oriented electrical steels. In cube-on-edge oriented electrical steel, the body-centered cubes making up the grains or crystals are oriented in a cube-on-edge position, designated (110) [001] in accordance with Miller's Indices.
Cube-on-edge oriented silicon steels are well known in the art and are commonly used in the manufacture of cores for transformers and the like. Cube-on-edge electrical steels are produced by a number of routings typically involving one or more operations of cold rolling and one or more operations of annealing, so as to obtain a cold-rolled strip having a commercial standard thickness. After the cold rolling is completed, the strip may be subjected to a decarburizing anneal and coated with an annealing separator. Thereafter, the strip is subjected to a high temperature final anneal at a temperature of about 1200.degree. C. As used herein and in the claims, the term "high temperature final anneal" refers to that anneal during which the cube-on-edge texture is produced as the result of secondary grain growth. The now-oriented electrical steel strip has its easiest axis of magnetization in the rolling direction of the strip so that it is advantageously used in the manufacture of magnetic cores for transformers and the like.
Various specific routings devised in recent years by prior art workers have resulted in cube-on-edge grain oriented silicon steels having markedly improved magnetic characteristics. As a consequence, such electrical steels are now considered to fall into two basic categories.
The first category is generally referred to as regular grain oriented silicon steel and is made by routings which normally produce a permeability at 796 A/m of less than 1870 with a core loss at 1.7 T and 60 Hz of greater than 0.700 W/lb when the strip thickness is about 0.295 mm.
The second category is generally referred to as high-permeability grain oriented silicon steel and is made by routings which normally produce a permeability at 796 A/m of greater than 1870 with a core loss less than 0.700 W/lb (at 1.7 T and 60 Hz) when the strip thickness is about 0.295 mm.
U.S. Pat. No. 3,764,406 is typical of those which set forth routings for regular grain oriented silicon steel. For regular grain oriented silicon steel, a typical melt composition by weight percent may be stated as follows:
C: less than 0.085% PA0 Si: 2%-4% PA0 S and/or Se: 0.015%-0.07% PA0 Mn: 0.02%-0.2% PA0 Si: 2%-4% PA0 C: &lt;0.085% PA0 Al (acid soluble): 0.01%-0.065% PA0 N: 0.003%-0.010% PA0 Mn: 0.03%-0.2% PA0 S: 0.015%-0.07%
The balance is iron and those impurities incident to the mode of manufacture.
In a typical but non-limiting routing for regular grain oriented silicon steel, the melt may be cast into ingots and reduced to slabs, continuously cast in slab form or cast directly into coils. The ingots or slabs may be reheated to a temperature of about 1400.degree. C. and hot rolled to hot band thickness. The hot rolling step may be accomplished without reheating, if the ingot or slab is at the required rolling temperature. The hot band is annealed at a temperature of about 980.degree. C. and pickled. Thereafter, the silicon steel may be cold rolled in one or more stages to final gauge and decarburized at a temperature of about 815.degree. C. for a time of about 3 minutes in a wet hydrogen atmosphere with a dew point of about 60.degree. C. The decarburized silicon steel is thereafter provided with an annealing separator, such as a coating of magnesia, and is subjected to a final high temperature box anneal in an atmosphere such as dry hydrogen at a temperature of about 1200.degree. C. to achieve the desired final orientation and magnetic characteristics.
U.S. Pat. Nos. 3,287,183., 3,636,579; 3,873,381; and 3,932,234 are typical of those teaching routings for high-permeability grain oriented silicon steel. A non-limiting exemplary melt composition for such a silicon steel may be set forth as follows in weight percent:
The above list includes only the primary oonstituents; the melt may also contain minor amounts of copper, phosphorus, oxygen and those impurities incident to the mode of manufacture.
In an exemplary, but non-limiting, routing for such high-permeability grain oriented silicon steel, the steps through the achievement of hot band thickness can be the same as those set forth with respect to regular grain oriented silicon steel. After hot rolling, the steel strip is continuously annealed at a temperature of from about 850.degree. C. to about 1200.degree. C. for from about 30 seconds to about 60 minutes in an atmosphere of combusted gas, nitrogen, air or inert gas. The strip is thereafter subjected to a slow cooling to a temperature of from about 850.degree. C. to about 980.degree. C., followed by quenching to ambient temperature. After descaling and pickling, the steel is cold rolled in one or more stages to final gauge, the final cold reduction being from about 65% to about 95%. Thereafter, the steel is continously decarburized in wet hydrogen at a temperature of about 830.degree. C. for about 3 minutes at a dew point of about 60.degree. C. The decarburized silicon steel is provided with an annealing separator such as magnesia and is subjected to a final box anneal in an atmosphere of hydrogen at a temperature of about 1200.degree. C.
It is common practice, with respect to both types of grain oriented silicon steels, to provide an insulative coating having a high dielectric strength on the grain oriented silicon steel (in lieu of, or in addition to, a mill glass). The coating is subjected to a continuous anneal at a temperature of about 815.degree. C. for about 3 minutes in order to thermally flatten the steel strip and to cure the insulative coating. Exemplary applied insulative coatings are taught in U.S. Pat. Nos. 3,948,786; 3,996,073; and 3,856,568.
The teachings of the present invention are applicable to both types of grain oriented electrical steels.
The pressure of increasing power costs has demanded that the materials used for transformer cores and the like have the lowest core loss possible. Prior art workers have long addressed this problem and have devised a number of methods (both metallurgical and non-metallurgical) to reduce the core loss of grain oriented electrical steels.
For example, from a metallurgical standpoint it is commonly known that core loss of oriented electrical steels can be decreased by increased volume resistivity, reduced final thickness of the sheet, improved orientation of the secondary grains, and by decreased size of the secondary grains. However, the process of secondary grain growth is neither well understood nor well controlled, often resulting in less than optimum control of the grain size and crystal texture, making it difficult to obtain grain oriented electrical steels having core losses closer to the theoretical limits. This problem is especially pronounced in those processes used to make high-permeability cube-on-edge grain oriented electrical steels, wherein larger than optimum secondary grain size is obtained. These circumstances have led a number of prior art workers to seek various non-metallurgical methods to improve core loss after the metallurgical processing is substantially complete.
One non-metallurgical approach is to apply a high-stress secondary coating onto the finished grain oriented electrical steel, as taught in U.S. Pat. No. 3,996,073. Such coatings place the grain oriented electrical steel strip in tension, which causes a decrease in the width of the 180.degree. magnetic domains and the reduction of the number of supplementary domains. Since narrow 180.degree. domains and few supplementary domains are desired in order to decrease the core loss of grain oriented electrical steels, such high-stress coatings are beneficial. However, the amount of tension or force that can be applied by these means is limited.
Another non-metallurgical approach is that of inducing controlled defects which is, in a sense, the creation of a substructure to limit the width of the 180.degree. domains in the finished grain oriented electrical steel. A basic technique is taught in U.S. Pat. No. 3,647,575 wherein the finished grain oriented electrical steel is provided with narrowly spaced shallow grooves or scratches transverse the rolling direction and on opposite sides of the sheet. While a decrease in core loss is realized by this method, the insulative coating is damaged and the steel sheet is characterized by an uneven surface. These factors will result in increased interlaminar losses and decreased space factor, respectively, in a transformer fabricated from a steel so treated.
U.S.S.R. Author's Certificate No. 524,837 and U.S.S.R. Pat. No. 652,230 disclose other methods to induce artificial boundaries in a finally annealed grain oriented electrical steel by localized deformation resulting from bending or rolling and localized deformation resulting from a high energy laser treatment, respectively. The application of these methods result in the desired improvement in the core loss of the electrical steel sheet after a subsequent anneal. Nevertheless, these methods cannot be advantageously used because of damage to the integrity of the insulative coating and the sheet flatness which result from these treatments.
U.S. Pat. Nos. 4,203,784 and 4,293,350 disclose other methods wherein the finally annealed grain oriented electrical steel sheet is provided with artificial boundaries by inducing very fine linear strains resulting from scribing the surface of the sheet with either a roller or a pulsed laser. These methods have been advantageously employed to reduce the core loss of grain oriented electrical steels. However, the methods taught in these two references are limited to stacked core transformer designs where the transformer core is not annealed to relieve the stresses resulting from fabrication. The slight dislocation substructure induced by the methods of these two references will be removed upon annealing above from about 500.degree. C. to about 600.degree. C., while typical stress relief annealing is done at about 800.degree. C. The damage done to the insulative coating (e.g., a mill glass, an applied coating, or both), even though less than by some other methods, is nontheless undesirable since very high interlaminar resistivity and coating integrity are desired for grain oriented electrical steels used in stacked core designs.
European Pat. No. 33878 teaches a method of laser treating according to U.S. Pat. No. 4,293,350, followed by a coating operation and heating the laser treated and coated sheet to about 500.degree. C. to cure the coating. However, this technique necessitates additional processing steps and expense, and the improvement to the material will not withstand an anneal in excess of 600.degree. C.
Co-pending application Ser. No. 403,714, filed July 30, 1982, in the names of Gary L. Neiheisel and Jerry W. Schoen, and entitled LASER TREATMENT OF ELECTRICAL STEEL teaches the treatment of magnetic materials of the type having domains of such size that refinement thereof would produce significant core loss improvement by a continuous wave laser. The magnetic material is scanned by the beam of the continuous wave laser across its rolling direction so as to subdivide the magnetic domains without damage to the insulative coating, resulting in improved core loss. Again the improvement to the material will not survive an anneal in excess of 600.degree. C.
The present invention is based upon the discovery that magnetic materials having domains of such size that refinement thereof would produce significant core loss improvement can have artificial boundaries induced therein by local heat treatments employing radio frequency induction heating or resistance heating either by radio frequency resistance heating or by treatment with an electron beam, followed by an anneal. The resulting magnetic material not only is characterized by improved core loss, but also its insulative coating (if present) and its flatness are unimpaired. Furthermore, the artificial boundaries will survive any subsequent anneal. The process of the present invention is potentially safer and easier to maintain than a laser system, and is more energy efficient.